Demodulator in a facsimile system

ABSTRACT

A facsimile receiver includes a demodulator system for converting a received MH code (Modified Huffman Code) signal into a signal representing actual run length information. The demodulator system comprises a demodulation table ROM for comparing the received MH code signal with MH codes stored therein. The leading &#34;0&#34; included in the received MH code signal is counted to provide a portion of an input data to be applied to the demodulation table ROM. When the received MH code signal relates to the terminating code and the input data to the demodulation table ROM becomes identical with one of MH codes stored in the demodulation table ROM, an output data of the demodulation table ROM is utilized without modification to obtain the actual run length information. When the received MH code signal relates to the makup code and the input data to the demodulation table ROM becomes identical with one of the MH codes stored in the demodulation table ROM, six lower bit zero&#39;s are added to the output data of the demodulation table ROM to obtain the actual run length information.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a demodulator system in a facsimileand, more particularly, to a demodulator system for converting areceived MH code (Modified Huffman Code) signal to an RL code (RunLength Code) signal in a facsimile system.

In a facsimile system, a transmitter forms the RL code signal inaccordance with the picture information read out from an original. TheRL code signal is modulated, at the transmitter, to the MH code signalwhich is suited for the compressed transmission, and the thus obtainedMH code signal is transmitted to a receiver. The receiver includes ademodulator for converting the received MH code signal to the RL codesignal, whereby the picture corresponding to the original is formed atthe receiver.

The MH code signal represents the RL information of zero through 1728bits. More specifically, when the run length is zero through 63 bits, aspecific MH code is assigned to each run length. This is referred to asthe terminating code. When the run length is 64 through 1728 bits, aspecific MH code is assigned to each group of 64 bits. That is, aspecific MH code is determined every increment of the run length by 64bits. This is referred to as the makeup code. The terminating code andthe makeup code are combined to represents the run length up to 1728bits. For example, the black of run length=2 is represented as "11" inthe MH code. The black of run length=1728 is represented as"0000001100101" in the MH code. That is, the MH code signal has bits twothrough thirteen.

Accordingly, the demodulator ROM must have thirteen address input bits.Therefore, the demodulator ROM must be of the capacity of 2¹³ =8K words.Alternatively, it is proposed to employ a plurality of PLA's(programmable logic array) which has a large number of input bits fordemodulating purposes. However, this is not practical since the PLA isevery expensive as compared with ROM.

Accordingly, an object of the present invention is to provide a noveldemodulator system for a facsimile system.

Another object of the present invention is to provide a demodulatorsystem for converting a received MH code signal into an RL code signalin a receiver of a facsimile system.

Still another object of the present invention is to provide ademodulator system for converting a received MH code signal into an RLcode signal, which requires an ROM of a small capacity.

Other objects and further scope of applicability of the presentinvention will become apparent from the detailed description givenhereinafter. It should be understood, however, that the detaileddescription and specific examples, while indicating preferredembodiments of the invention, are given by way of illustration only,since various changes and modifications within the spirit and scope ofthe invention will become apparent to those skilled in the art from thisdetailed description.

In accordance with the present invention, by properly shifting thereceived MH code signal, the demodulator can be formed by an ROM of 2Kword capacity.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from the detaileddescription given hereinbelow and the accompanying drawings which aregiven by way of illustration only, and thus are not limitative of thepresent invention and wherein:

FIG. 1 is a schematic block diagram of an embodiment of a facsimilereceiver of the present invention; and

FIG. 2 is a block diagram of a demodulator system included in thefacsimile receiver of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically shows a facsimile receiver employing an embodimentof a demodulator system of the present invention.

The facsimile receiver mainly comprises a central processor unit (CPU) 1for developing various control signals for controlling the operation ofthe facsimile receiver. A memory 2 is provided for storing a data anddeveloping the data in accordance with the control signals derived fromthe CPU 1. An MH code (Modified Huffman Code) signal transmitted from atransmitter and received by the facsimile receiver is introduced into ashift register 3. When the MH code signal is introduced into the shiftregister 3, a control logic 9 functions to transfer the contents storedin the shift register 3 into a read only memory (ROM) 4, which containsa demodulation table, in response to a clock signal φ. An MH codedetection flip-flop 7 is connected to the ROM 4 for sampling aparticular one bit, for example, the eighth bit of an output signal ofthe ROM 4 while the MH code is introduced into the ROM 4.

While the MH code is not yet detected, four bits of the output signal ofthe ROM 4 (fourth through seventh bits) are latched in a large circuit8, and the thus latched data is applied to the ROM 4 at the followingclock timing. When the MH code is detected, a detection signal isdeveloped from the MH code detection flip-flop 7 for conducting theinterruption on the operation of the CPU 1. Upon development of thedetection signal, the CPU 1 reads out the output data from the ROM 4 viaa buffer 5. In the case when the MH code is the terminating code, thedata is applied directly to an RL (run length) register 6. In the casewhen the MH code is the makeup code, the data is modified to show theactual run length and, then, introduced into the RL register 6. The RLregister 6 counts up the run length number, and develops pictureinformation to a recorder indicative of black or white information.

The data processing operation in the demodulation table of the ROM 4 isas follows. As already discussed above, the MH code is classified intothe terminating code and the makeup code. In this example, both of theterminating code and the makeup code comprise eight bits to show the runlength information. The terminating code is related to the zero through63 bit information, which can be represented by 2⁶, and, therefore, theeight bit information can be used as the run length information withoutmodification. The run length represented by the makeup code has 2⁷through 2¹¹ length and, therefore, in the present system, the last sixbit "zero" is omitted. Instead thereof, information "1" is inserted inthe seventh bit for representing that the signal relates to the makeupinformation, the seventh bit being not used in the makeup code nor inthe terminating code.

For example, RL=1728 is represented by the makeup code. Morespecifically, the run length information should be "00011011000000." Inthis case, the data processed in the ROM 4 is "01011011." The eighth bitis assigned to the detection bit as to whether the received signal isthe MH code signal or not. The MH code information is sequentiallyintroduced into the demodulation table by shifting the information, andthe introduced information is compared with the table to determine as towhether the introduced information corresponds to any one of the MH codeprogrammed in the demodulation table.

Before the introduced information corresponds to any one of the MH code,the data is processed in the following manner. Every MH code isrepresented by information less than or equal to thirteen bits. Forexample, the information of black of RL=1728 is represented in the MHcode as follows:

    "0000001100101"

The first six bits are "0". It will be clear from the MH code table thatthe leading "0" occupies the first four bits at the least in the casewhere the MH code has the maximum thirteen bits. The leading "0"occupies the first eleven bits at the maximum when the MH coderepresents one line completion, namely, the line synchronization codeEOL (000000000001).

In this example, the ROM demodulation table has output terminals D₀through D₇. The above-mentioned leading "0" is counted up through theuse of three bit outputs D₆, D₅ and D₄ of the ROM. The count upoperation is terminated at the moment when the information "1" firstappears. As to the line synchronization code EOL, the output D₃ is usedin addition to the three bit outputs D₆, D₅ and D₄ for counting theleading "0". The count operation is terminated when the countinformation becomes eleven, and the EOL is detected at the subsequentappearance of the information "1".

The eighth bit output D₇ of the ROM is assigned to the detection bit forthe MH code. Until the MH code detection is conducted, the eight bit D₇develops a signal of logic "1." The remaining three bit outputs D₀, D₁and D₂ are redundancy bits. In this embodiment, the three bit outputsD₀, D₁ and D₂ are "000" while the leading "0" is counted, and used tocount up the bit number after the information "1" appears at the MH codeinput.

The demodulator system of the present invention comprises an ROM havingten bit input terminals. The MH code input information is applied to sixbit terminals, and the remaining four bit input terminals are used forfeeding-back purposes through the latch circuit. Accordingly, theseventh bit and the eight bit data of the MH code is formed through theuse of the data fed-back through the latch circuit in accordance with aparticular program. For example, when the MH code informationcorresponding to black of RL=45 is introduced, the input data A₀ throughA₉ and the output data D₀ through D₇ of the ROM are varied as shown inthe following TABLE I until the MH code detection is performed. The mHcode assigned to black of RL=45 is "000001010101."

                                      TABLE I                                     __________________________________________________________________________    (Black of RL = 45)                                                            A.sub.9                                                                         A.sub.8                                                                         A.sub.7                                                                         A.sub.6                                                                         A.sub.5                                                                         A.sub.4                                                                         A.sub.3                                                                         A.sub.2                                                                         A.sub.1                                                                         A.sub.0                                                                           D.sub.7                                                                         D.sub.6                                                                         D.sub.5                                                                         D.sub.4                                                                         D.sub.3                                                                         D.sub.2                                                                         D.sub.1                                                                         D.sub.0                                   __________________________________________________________________________    0 0 0 0 0 0 0 0 0 0 t.sub.1                                                                         1 0 0 1 0 0 0 0                                         0 0 1 0 0 0 0 0 0 0 t.sub.2                                                                         1 0 1 0 0 0 0 0                                         0 1 0 0 0 0 0 0 0 0 t.sub.3                                                                         1 0 1 1 0 0 0 0                                         0 1 1 0 0 0 0 0 0 0 t.sub.4                                                                         1 1 0 0 0 0 0 0                                         1 0 0 0 0 0 0 0 0 0 t.sub.5                                                                         1 1 0 1 0 0 0 0                                         1 0 1 0 0 0 0 0 0 1 t.sub.6                                                                         1 1 0 1 0 0 0 1                                         1 0 1 0 0 0 0 0 1 0 t.sub.7                                                                         1 1 0 1 0 0 1 0                                         1 0 1 0 0 0 0 1 0 1 t.sub.8                                                                         1 1 0 1 0 0 1 1                                         1 0 1 0 0 0 1 0 1 0 t.sub.9                                                                         1 1 0 1 0 1 0 0                                         1 0 1 0 0 1 0 1 0 1 t.sub.10                                                                        1 1 0 1 0 1 0 1                                         1 0 1 0 1 0 1 0 1 0 t.sub.11                                                                        1 1 0 1 1 1 1 0                                         1 0 1 1 0 1 0 1 0 1 t.sub.12                                                                        0 0 1 0 1 1 0 1                                         __________________________________________________________________________

During the time period t₁ through t₅, the MH code signal is sequentiallyshifted and the upper bits "0" are introduced. The leading "0" iscounted and the count result is developed at the output terminals D₆through D₃. The thus developed output signal is fed back to the inputterminals A₉ through A₆, and introduced at the subsequent shiftoperation. The output D₇ bears the logic "1" since the MH code detectionoperation is performed. At the time t₆, information "1" is applied tothe input terminal A₀. The count operation of the leading "0" isterminated, and instead thereof, the subsequent bit number is countedthrough the use of the outputs D₀ through D₂. Till the time t₁₁, thedata is sequentially shifted and applied to the data input side. Asalready discussed above, the address input is not applied to the seventhbit and the eighth bit. Therefore, the information must be modifiedthrough the use of the data applied from the output terminals D₆ throughD₃. More specifically, when the seventh bit data and the eighth bit datain the MH code are different from the contents applied from the outputsD₆ through D₃, the outputs D₆ through D₃ are modified in accordance withthe program and, then, applied to the address inputs.

In the case of black of RL=45, the outputs D₆ through D₃ at the time t₁₀are "1010." If the outputs "1010" are applied to the address inputs ofthe ROM at the time t₁₂, the contents for the input A₆ become "0," whichdiffer from the MH code for black of RL=45, "000001010101." Therefore,in the present invention, at the time t₁₁, in accordance with theprogrammed contents, the output D₃ is modified to "1," and the thusmodified data "1011" is fed back to the input side. In this way, the ROMdetects that the applied signal is the MH code signal for the black ofRL=45, and develops "101101" at the output terminals D₆ through D₀,which represents RL=45. At the moment when the above MH code isdetected, the detection bit D₇ changes to "0."

In the case where the MH code detection is not achieved, namely, when anerroneous data is received by the facsimile receiver, the MH detectionbit D₇ is changed to "0" at a time other than the MH code detectionperiod, whereby a different RL is forced to be detected to release thesystem from the detection impossible condition.

The demodulator system in the facsimile receiver of FIG. 1 is shown, indetail, in FIG. 2. In this example, two 1K×8 bit ROM's are employed forthe demodulation table, one being assigned to the black information, andthe other being assigned to the white information. The operation of oneROM 4-1 is only discussed hereinbelow for the purpose of simplicity.

The facsimile signal transmitted from the transmitter includes a signalfor indicating the initiation of one line. When the line initiationsignal is detected, the CPU 1 (FIG. 1) develops a line start signal LCLRto reset the demodulator system in the facsimile receiver. The thusobtained reset signal is applied to shift registers 3-1, 3-2, the latchcicuit 8 and the two ROM's 4-1, 4-2 to clear them. The reset signal isalso applied to a black/white selection flip-flop 9-10 to reset theflip-flop, whereby the black/white selection flip-flop 9-10 is placed ina condition selecting the "white". Then, the demodulating operation isinitiated.

The CPU 1 functions to supply the first shift register 3-1 with eightbit MH data through data lines D_(A) through D_(H). More specifically,the transmitted MH data is supplied to the first shift register 3-1eight bits by eight bits. This writing-in operation is controlled by awrite-in signal OTOB which is applied to the first shift register 3-1.The write-in signal OTOB is also applied to reset terminals R₀₁ and R₀₂of an eight bit counter 9-3. The eight bit counter 9-3 is held in thereset state during the writing-in operation. The eight bit counter 9-3is released from the reset state when the writing-in operation iscompleted to turn on a clock gate 9-1 through gates 9-4 and 9-5, wherebythe clock φ is introduced into the eight bit counter 9-3 for countingpurposes. The counting operation in the eight bit counter 9-3 iscontrolled by the on/off operation of the gate 9-1. The gate 9-1 isturned off either by a signal applied from the flip-flop 7 through thegate 9-5, the signal being developed when the MH code detection isachieved, or by a QH signal applied through the gates 9-4 and 9-5, theQH signal being developed with eight bit count is completed.

While the clock φ is counted by the eight bit counter 9-3, the datastored in the first shift register 3-1 is shifted in response to theclock φ and transferred to the second shift register 3-2 from an outputterminal QH in response to a clock pulse φ. The second shift register3-2 includes six bit parallel outputs Q_(A) through Q_(F), which areapplied to the lower six bit inputs A₀ through A₅ of the ROM's 4-1 and4-2. The ROM's 4-1 and 4-2 are alternatively enabled in accordance withthe output signal derived from the black/white selection flip-flop 9-10.The eighth bit output D₇ of the ROM's 4-1 and 4-2 is applied to the MHdetection flip-flop 7 and sampled in response to an MH detection clockdeveloped from an output terminal 1Q of a mono-stable multivibrator9-11. The Q output of the MH detection flip-flop 7 functions as a timingsignal for effecting the interruption to the CPU 1, whereby the contentsstored in the ROM are applied to the RL register (6) when the MH code iswritten into the ROM.

While the MH code information is sequentially shifted and introducedinto the ROM and when the MH detection is not yet achieved, the latchcircuit 8 latches the four bit outputs D₆ through D₃ of the ROM inresponse to a latch clock φ_(L). The thus latched contents are appliedto the address inputs A₆ through A₉ of the ROM at the subsequent clockφ. The data shift to the ROM is repeated in response to the clock signaluntil the introduced data becomes identical with any one of programmedMH codes. When the address input information becomes identical with anyone of the MH codes, the detection output is developed at the eighthoutput terminal D₇ to perform the interruption of the CPU 1.

When the interruption command is applied to the CPU 1, the CPU 1develops a read-out signal IPOB to turn on the buffer 5, thereby readingout the contents stored in the ROM 4-1 or 4-2. If the read out MH codeis the terminating code, the run length can be represented by six bitsand, therefore, the ROM contents are applied to the RL register 6without modification. If the read out MH code is the makeup code, theROM outputs are modified to obtain the actual run length information.More specifically, the seventh bit "1" of the read out RL information isreset, and the information "0" is added to the last six bits. The thusobtained actual run length information is transferred to the RL register6.

The read-out signal IPOB is also applied to the mono-stablemultivibrator 9-11 for detecting the trailing edge of the read-outsignal IPOB. An output 2Q is applied to the second shift register 3-2,the latch circuit 8 and the flip-flop 7 to clear them. The read-outsignal IPOB is further applied to the black/white selection flip-flop9-10 to control the selection of the two ROM's 4-1 and 4-2.

In the case where the RL information read out from the ROM is the makeupcode, the terminating code subsequently appears. If the makeup coderepresents the black information, the subsequent terminating code alsorepresents the black information. In this case, the seventh and sixthbit outputs D₆ and D₅ are "10" and, therefore, information "00" isapplied to the black/white selection flop-flop 9-10 through gates 9-13,9-14 and 9-15. The black/white selection flip-flop 9-10 is not reversedand, therefore, the Q output is not changed to introduce the subsequentMH code information into the same ROM. In the case where the terminatingcode is read out, the black/white selection flip-flop 9-10 receives aninput signal "11" and, therefore, the Q output is reversed. That is, theROM 4-1 and the ROM 4-2 are alternatively selected upon every read outoperation of the terminating code.

In the foregoing embodiment the two 1K word ROM's are employed. However,one 2K word ROM can be employed without varying the circuitconstruction.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications are intended to be included within the scope of thefollowing claims.

What is claimed is:
 1. A demodulator system for a facsimile receivercomprising:a programmed demodulation table for converting received MHcode (Modified Huffman Code) information into run length information;input means for sequentially shifting said received MH code informationand applying an input data to said programmed demodulation table;feed-back means for feeding back a portion of an output data from saidprogrammed demodulation table to said programmed demodulation table as aportion of the input data thereto until said input data corresponds toany one of MH codes programmed in said programmed demodulation table;and output means for forming run length information through the use ofthe output data derived from said programmed demodulation table when theinput data applied to said programmed demodulation table becomesidentical with any one of MH codes programmed in said programmeddemodulation table.
 2. The demodulator system of claim 1, furthercomprising:counter means for counting a leading "0" included in thereceived MH code information; and transfer means for applying contentsstored in said counter means to said programmed demodulation table,thereby obtaining said portion of the output data for feed-backpurposes.
 3. The demodulator system of claim 1 or 2, wherein saidprogrammed demodulation table comprises a read only memory having eightbit output terminals.
 4. The demodulator system of claim 3 furthercomprising detection means for detecting whether the received MH codeinformation relates to terminating code information or makeup codeinformation.
 5. The demodulator system of claim 4, wherein said outputmeans forms the run length information without modifying the output dataderived from said programmed demodulation table when said detectionmeans indicates that the received MH code information relates to theterminating code information, and said output means forms the run lengthinformation by adding six lower bit zero's to the output data derivedfrom said programmed demodulation table when said detection meansindicates that the received MH code information relates to the makeupcoded information.